The most commonly used high pressure hydraulic coupling comprises a swivelling nut on one of the coupling parts and a threaded boss on the other. This type of coupling has a number of disadvantages. A main disadvantage is that space is required for turning the nut with a spanner. Another major disadvantage is that the coupling may be damaged when being used by unskilled labour. Typically the nut may be tightened too much, stripping the thread, may not be tightened enough so that there is not an adequate seal or may be put on skew. A further disadvantage is that the coupling parts cannot turn relative to one another when they are connected. Thus, the hoses must be so arranged that they can accomodate the twist in the hoses when the parts connected by the hose move relative to one another. Finally, considerable time is required for tightening and untightening the coupling parts and this time is increased when there is limited space for moving the spanner or other tightening tool.
Another type of coupling which is commonly known as the "quick coupling" obviates many of these drawbacks. This coupling comprises two parts which are connected together by steel balls which span the space between them. Connecting and disconnecting the two parts can be relatively easily carried out. However, this quick coupling has many serious disadvantages. First the cost of such a coupling is very high due to its complicated design. Second, in practice it has been found to be very sensitive to dirt obstructing the movement of the locking steel balls and sleeves. Third, these couplings malfunction when used in a high pressure hydraulic system and, in particular, such a system where there are high pressure shock loads. The reason for this is that the area of the locking system that has to carry these loads is very small. For these reasons, the quick coupling is less used than the first mentioned coupling.
A third type of coupling is that known as the staple lock coupling. One such coupling is illustrated in Br. Pat. Nos. 1,066,936 and 1,253,981. This type of coupling is fairly new on the market and is gaining popularity. However, this type of coupling also has disadvantages. The unit is usually big and clumsy. The locking staple can only be inserted from a few (usually only four) directions. The cost of manufacture is not insubstantial because internal grooves must be cut on the female member. Most seriously, however, the area on the staple that has to carry the load is rather small, and under high pressure shock conditions deformations occur that result in malfunction.